Process for decomposing a polymer to its monomer or monomers

ABSTRACT

A process for decomposing a polymer which is capable of undergoing thermal depolymerization to its monomer or monomers, such as for example poly(methylmethacrylate), and for the recovery of at least one of the monomers, includes the steps of subjecting the polymer in solid, gel, partially molten or molten form to microwave heating for a time and at a temperature sufficient to decompose the polymer to produce the monomer or monomers in gaseous, liquid or solid form, without substantial decomposition of the monomer or monomers, and recovering at least one of the monomer or monomers. The monomer or monomers may then be reused for plymerisation.

BACKGROUND OF THE INVENTION

This invention relates to a process for decomposing a polymer which iscapable of undergoing thermal depolymerization to its monomer ormonomers, and for recovery of at least one of the monomers, whichmonomer may be recycled to a polymerization process.

The conventional decomposition or depolymerization process for thedepolymerization of waste poly(methyl metharcylate) ("PMMA") makes useof a lead bath reactor. The waste PMMA is crushed into chips (1 to 5 cmin diameter) and decomposed on the surface of the molten lead bath. Themolten lead is maintained at 520 to 550° C. by means of a diesel burnerwhich operates at about 900° C. In the reactor, depolymerization takesplace producing a gaseous product which is (methyl methacrylate) monomer("MMA"), which is condensed in a condenser, with a solid dross or ashremaining on the surface of the lead bath. The dross is composed mainlyof carbonaceous material, lead (40-60% m/m Pb) and inorganic residuesfrom pigments and additives. The crude monomer (approximately 85% MMA)may then be purified as follows. The monomer is washed in a 9% causticsolution (containing 40 ppm copper sulphate heptahydrate CuSO₄.7H₂ O)which removes any traces of lead. The washings (containing approximately12 ppm lead) are treated and discharged to a slimes dam, but represent apotential environmental hazard. The final purification step is a vacuumdistillation at 65° C., to remove heavy organic impurities. This type ofdepolymerization reactor may be operated to produce refined monomer withan average purity of 99.3% and yield of 85%.

This type of depolymerization reactor has numerous disadvantages. Themost significant disadvantage of the lead bath reactor is theenvironmental and safety hazard associated with lead. Anotherdisadvantage of the reactor is that it has to be operated on anon-continuous basis. Generally, the reactor is shut down afterapproximately five days operation due to fouling on the surface causedby the dross, which inhibits heat transfer from the lead to the PMMA.The cooling and cleaning operation may result in one co two daysdowntime. A further disadvantage is that the lead containing dross isgenerated as a waste product. Lead must be recovered from the dross,which then needs to be disposed of in an environmentally suitablemanner. The lead bath subsequently must be reheated to operatingtemperature. Thus, this process is very energy inefficient.

In an article entitled Polymethyl Methacrylate Binder Removal from anAlumina Compact:Microwave versus Conventional Heating in Mat. Res. Soc.Symp. Proc. vol. 269, 1992 by moore et al, there is disclosed thatcompact samples of alumina and polymethyl methacrylate were heated in a2450 MHz microwave cavity and by conventional heating in an electricfurnace. Various heating schedules were used to effect the removal ofthe polymeric binder by thermal decomposition. Dielectric properties,porosity and other physical properties were investigated in order tobetter understand the binder removal process in a microwave field.Results of the study emphasized the amount of the carbon residualsremaining in the bulk. In this article it is stated that PMMA decomposesinto monomer, water. benzene and other components. These components arethen further decomposed to hydrocarbons between 500° C. and 1000° C. Inother words, the PMMA is completely decomposed into hydrocarbons.

In an article entitled Microwave reactions of polyethyleneterephthalate, in Polym.Mater.Sci.Eng, 71,531-2, 1994, by Gilmer et al,there is disclosed that a glass reactor was designed for the microwavecavity to allow conventional chemical reactions to be carried out withfocused microwave heating. Employing this setup for reactions concerningthe synthesis and depolymerization of poly(ethylene terephthalate)(PET), in general, the monomers for PET are not good microwave heaters(absorbers), with the possible exception of ethylene glycol (EG). In thedepolymerization of PET by EG and the reaction of EG with di-methylterephthalate to form bis (hydroxyethyl) terephthalate and methanol(MeOH), identical reaction rates were obtained using either thermal ormicrowave heating.

In other words, this article discloses in general the depolymerizationof PET, in a solvent, i.e EG.

There is thus a need for a new process for decomposing a polymer to itsmonomer or monomers, which process makes use of an economic, safe andenvironmentally friendly thermal decomposition route which eliminatesthe use of lead, and which results in a lesser amount of "ash" or"dross" which is free of contamination by lead.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a processfor decomposing a polymer which is capable of undergoing thermaldepolymerization to its monomer or monomers, the polymer being selectedfrom the group consisting of poly(methyl methacrylate),polyterrafluoroethylene, polystyrene, poly(ethylene cerephthalate),poly(α-methylstyrene) and polyisobutylene, and for recovery of at leastone of the monomers, which includes the steps of:

(i) subjecting the polymer in solid, gel, partially molten or moltenform to microwave heating for a time and at a temperature sufficient todecompose the polymer to produce the monomer or monomers in gaseous,liquid or solid form, without substantial decomposition of the monomeror monomers; and

(ii) recovering at least one of the monomer or monomers.

The polymer may be any suitable polymeric material which is capable ofundergoing thermal depolymerization to its corresponding monomer ormonomers, in reasonable yields, and which is selected from the listgiven above.

The polymer to be decomposed may be present as a single polymer, or in amixture of two or more polymers. In this latter case, both or all thepolymers may be as listed above, or one polymer may be as listed above,and the other polymer or polymers may be different polymers which may ormay not decompose to their monomer or monomers.

The process of the invention is of particular application to poly(methylmethacrylate) which undergoes thermal depolymerization to methylmethacrylate, in up to 99% monomer yield.

The process of the invention may include the following steps:

(iii) where two or more monomers are recovered in step (ii), separatingthe monomers from one another;

(iv) where the monomer or monomers are in gaseous form, condensing themonomer or monomers; and

(v) purifying the condensed monomer or monomers.

Thereafter, the monomer or monomers may, for example, be recycled to aprocess for polymerization of the monomer or monomers to produce thepolymer, or other polymers, or may be used to manufacture otherproducts.

Prior to step (i), the polymer may be preheated to a suitabletemperature. The heating may be carried out in any conventional manner.

In step (i), the polymer may be mixed with a microwave absorber orsusceptor, i.e. a material with a high dielectric loss factor. Examplesof suitable microwave absorbers or susceptors are carbon powder (carbonblack, furnace black, lampblack); FREQUON B20 which is a fine whiteinorganic powder; M(O₃ ZO_(x) R)_(n) where M=metal, Z=group V atoms withmolecular weight ≧30, x=0.1, R=H or organic radical. n=1.2, for exampleZr(O₃ PCH₂ CH₂ SH); high loss ceramics, such as silicon carbide (SiC);ferrites; most electro-ceramics (e.g. barium titanate, BaTiO₃); andalkali metal oxide-based materials (e.g. sodium oxide). The use of amicrowave absorber or susceptor may enhance the decomposition process,in polymers which themselves do not interact or interact only poorlywith microwaves (such as non-polar materials.).

Step (i) may be carried out under an inert atmosphere, for example anitrogen atmosphere.

In step (i), the microwave heating may be carried out in a mono-mode, amulti-mode or a non-resonant cavity of a microwave reactor, at anysuitable microwave frequency, and preferably at 2.45 GHz or 915 MHz,which are industrial frequencies.

The process of the invention may be carried out on a continuous or on abatch basis.

According to a second aspect of the invention there is provided the useof a monomer produced by the process described above in a process forthe polymerization of the monomer, optionally with one or moreadditional monomers, and optionally mixed with virgin monomer in asuitable ratio, to produce a polymer, and also to the process ofpolymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a laboratory scalemono-mode microwave apparatus for use in the process of the invention;

FIG. 2 is an expanded view of a portion of the mono-mode cavity of theapparatus of FIG. 1;

FIG. 3 is a diagrammatic cross-sectional view of a modification of themono-mode cavity of the apparatus of FIG. 1;

FIG. 4 is a diagrammatic cross-sectional view of a laboratory scalemulti-mode microwave apparatus for use in the process of the invention;

FIG. 5(a) is a diagrammatic view of a bench scale, mono-mode microwavereactor and apparatus for use in the process of the invention;

FIG. 5(b) is a diagrammatic view of a microwave variable power sourcefor use with the reactor of FIG. 5(a); and

FIG. 6 is a diagrammatic view of a multi-purpose (±1000 ml),non-resonant microwave reactor and apparatus for use in the process ofthe invention.

DESCRIPTION OF EMBODIMENTS

The crux of the invention is a process for decomposing a polymer whichis capable of undergoing thermal depolymerization to its monomer ormonomers, the polymer being selected from the group consisting ofpoly(methyl methacrylate), polytetrafluoroethylene, polystyrene,poly(ethylene terephthalate), poly(α-methylstyrene) and polyisobutylene,and for recovery of at least one of the monomers, which includes thesteps of:

(i) subjecting the polymer in solid, gel, partially molten or moltenform to microwave heating for a time and at a temperature sufficient todecompose the polymer to produce the monomer or monomers in gaseous,liquid or solid form, without substantial decomposition of the monomeror monomers; and

(ii) recovering at least one of the monomer or monomers.

As the purpose of the invention is to recover at least one of themonomers the microwave heating must be such that the monomer to berecovered is not substantially decomposed further. Thus, the phrase"without substantial decomposition of the monomer or monomers" must beinterpreted in this way.

It is to be noted that the polymer is subjected to the microwaves insolid, gel, partially molten or molten form. In other words, step (i) iscarried out in the absence of a solvent or solvents for the polymer.

By a "gel" there is meant a system which contains a range of productsfrom monomer through to the polymer, i.e a partially polymerizedmixture. The system contains no solvent for the polymer.

The process of the invention is of particular application to polymersselected from the group consisting of poly(methyl methacrylate),polytetrafluoroechylene, polystyrene, poly(α-methylstyrene) andpolyisobutylene, i.e excluding poly(ethylene terephthalate), sincedegradation of this polymer to recover both monomers, ethylene glycoland terephthalic acid, involves an hydrolysis reaction, which requiresthe presence of a suitable solvent such as ethylene glycol or anotherhydroxy containing species, for example methanol. In the process of thisinvention, only terephthalic acid may be recovered in the absence of asolvent.

The process of the invention is more especially of application topoly(methyl methacrylate).

As stated above, the polymer to be decomposed may be present as a singlepolymer or in a mixture of two or more polymers. In this latter case,both or all the polymers may be as listed above, or one polymer may beas listed above, and the other polymer or polymers may be differentpolymers which may or may not decompose to their monomer or monomers.

Thus the process may be used selectively to decompose and recover themonomer of one polymeric substance, or to decompose and recover two ormore monomers consecutively, from a co-polymer or physical mixture ofseveral polymers, where the decomposition temperatures and thedielectric loss factors are suitably different to allow this. In theprocess, the temperature of decomposition is controlled so as to preventsubstantial further degradation of the monomer or monomers toundesirable hydrocarbons or other carbonaceous materials.

As stated, the process of the invention is of particular application forthe depolymerization of acrylic resins and polymers, more particularlypoly(methyl methacrylate), or co-polymers and physical mixtures ofpoly(methyl methacrylate) with other polymeric substances.

The polymer may be treated in gel, partially molten or molten form, orin solid form in the form of chips, for example having a diameter offrom 0.1 to 5 cm, or in the form of a powder.

The process is of particular application for the depolymerization ofboth cast and extruded acrylic products and high impact acrylic productswhich may contain other recyclable (e.g. polystyrene) or non-recyclablecomponents.

It is also of use in the recycling of powdered waste PMMA and acrylicproducts, also known as swarf, which arises from the cutting of PMMA andacrylic products, and PMMA gel (partially polymerized PMMA/MMA mixture).

In step (i) the polymer may be mixed with a microwave absorber orsusceptor, i.e. a material with a high dielectric loss factor. Anexample of a suitable microwave absorber or susceptor is carbon black.Other examples are given above. Carbon black is a very efficientmicrowave absorber and heats up within seconds, enhancing thedecomposition of the polymer. The thermal energy is transferred from thecarbon black to the polymer resulting in rapid decomposition. Thepercentage by mass of the carbon black added may be from 0.5 to 50%,preferably from 0.5 to 5% by mass of the polymer. The carbon black isunaffected in the inert atmosphere and can therefore be recycled,provided that residual inorganic pigments do not interfere with theprocess.

It is not, however, always necessary to use a microwave absorber orsusceptor. By optimization of the cavity design (geometry anddimensions), coupling with the material to be heated is maximized,reducing the need for a susceptor. For example, in the case of PMMA,after optimization of the reactor design, no microwave absorber orsusceptor was required, since PMMA coupled efficiently with themicrowave energy.

Step (i) may be carried out under an inert atmosphere, e.g. a nitrogenatmosphere. The nitrogen atmosphere is used to sweep the gaseous productout of the reaction zone and to prevent ignition of hot volatile organicvapors, in the case where the monomer is released as a flammable organicliquid or vapors. The nitrogen also serves to prevent ignition of carbonwhere this is used as a microwave susceptor.

Further, in order co prevent the condensation of the gaseous products onthe walls of the microwave reactor, it is possible to heat the walls ofthe microwave reactor by conventional means.

Prior to step (i), the polymer may be preheated to a suitabletemperature, the heating being carried out in any conventional manner.

The reason for this is that it has been found that the microwaveabsorption efficiency of some polymers, such as PMMA, increases withincreased temperature. This property can be utillized to improve theoverall efficiency of the process of the invention. Thus, the polymermay be preheated conventionally so as to improve the microwaveabsorption efficiency and then at the elevated temperature, the polymeris irradiated with microwave energy. The advantages of the preheatinginclude the fact that the microwave absorption efficiency may beimproved through more effective coupling of the energy with the polymerat the raised temperature and, the overall energy cost of the processmay be reduced by hybrid heating, as the microwave heating is only usedin the most effective decomposition (temperature) zone.

When the polymer is PMMA, the PMMA may be preheated to a temperature offrom 80° C. to 150° C. inclusive. However, with appropriate design ofthe microwave apparatus, PMMA couples very effectively with themicrowave energy, and pre-heating is nor necessary.

The microwave heating may be carried out in a mono-mode, a multi-mode ora non-resonant cavity of a microwave apparatus at any suitable microwavefrequency as determined by dielectric property measurements and morepreferably at an industrial frequency such as 2.45 GHz or 915 MHz. Themicrowave irradiation directly interacts with and heats the polymer,causing the decomposition to take place.

The microwave energy may be generated by a magnetron or klystron usingelectrical power. The microwaves are generated in a microwave generatorand transferred into a mono-mode, or a multi-mode or a non-resonantcavity by means of a waveguide. Power requirements will depend on thedielectric properties of the material under irradiation at a particularfrequency, the mass of material, the feed rate and the systemefficiency.

The microwave apparatus preferably should allow for fine tuning of themicrowave irradiation applied to the cavity to maximize the electricfield density in the region of the polymer, and hence to improve theefficiency of depolymerization of the polymer.

The process may be carried out on a batch basis, or on a continuousbasis by continuously feeding the polymer into the microwave cavity and,in the case of a gaseous product, sweeping the gaseous product out ofthe reaction zone. The inert sweep gas may pass either co-currently orcounter-currently co the passage of the polymer. The process also may beoperated without a continuous inert gas sweep by suitable design of theequipment.

The residence time of the polymer under microwave irradiation will bedetermined so as to obtain the desired decree of decomposition of thepolymer, without substantial decomposition of the monomer occurring, andis also related to the type and mass of the polymer being decomposed thedielectric properties of the polymer, the feed rate, the power used, andthe efficiency of the system design. As stated above preheating thepolymer conventionally before microwave irradiation reduces theirradiation time required as the microwave absorption efficiency isimproved.

Similarly, the use of a susceptor reduces the required irradiation time,which is again dependent on the above factors in addition to the ratio(m/m) of polymer to susceptor.

In general, it is desirable to limit the maximum temperature within themicrowave apparatus so as to decompose the polymer to itsmonomer/monomers, without substantial further degradation of themonomer/monomers.

In the case of two polymers (i.e a mixture or co-polymer), in theabsence of a susceptor, the upper temperature is largely determined bythe polymer (I) with the highest dielectric loss factor, i.e that whichabsorbs or interacts most strongly with the microwave energy at aparticular frequency. An example of this is the case of a mixture ofPMMA and polystyrene, where the upper temperature is determined by thePMMA, due to its greater interaction with microwave energy. In thiscase, the maximum decomposition temperature is higher than the typicaltemperature range for decomposition of the second polymer (polystyrene).Provided that this temperature maximum does not exceed the temperatureat which further degradation of the monomer from decomposition of thesecond polymer (II) occurs, both monomers from polymers (I) and (II) maybe recovered, simultaneously. However, if the maximum temperature doesexceed the degradation temperature for the second monomer, only monomerfrom the first polymer (I) may be recovered. This applies similarly formixtures containing more than two polymers.

In the case of two or more polymers (i.e a mixture or co-polymer), inthe absence of a susceptor, where the upper or maximum temperature,defined by the polymer (I) with the highest dielectric loss factor, issufficiently below the temperature for decomposition of the secondpolymer (II) (and subsequent polymers, III, IV, etc.), then it may bepossible to selectively decompose the first polymer and recover themonomer from polymer (I) selectively, prior to decomposition of thesecond polymer (II) (and subsequent polymers, III, IV, etc.) andrecovery of the monomer (monomers). Invariably, some decomposition ofthe second and subsequent polymers may occur concomitantly with that ofthe first, to varying degrees, if the temperature ranges ofdecomposition overlap with that of the first polymer. Where the secondpolymer (II) (and/or subsequent polymers, III, IV, etc) does (do) notinteract or interacts (interact) only poorly with microwaves, it may benecessary to add a suitable susceptor material to the reactor after thedecomposition of the polymers which interact strongly with microwaves,in order to increase the temperature for further decomposition of theremaining polymers.

Generally, with PMMA, decomposition begins at around 300° C. and iscomplete by about 400° C. Further heating above 400° C. results inundesirable charring caused by the further decomposition of monomer toundesirable carbonaceous products.

Thus, it is essential to control temperature in the case of PMMAcarefully and to prevent substantial increase in temperature above 400°C. This is a further reason for periodic removal of inorganic pigmentresidues, which may increasingly absorb microwave energy onaccumulation, leading to thermal runaway and substantial charring of theproduct and reduced monomer purity and recovery.

In the case of polystyrene, the temperature range of decompositionshould be limited to about 230° to about 400° C.

In the case of poly(ethylene terephthalate), the temperature range ofdecomposition should be limited to about 300° C. to about 450° C.

In the case of polytetrafluoroethylene, the temperature range ofdecomposition should be limited to about 450° C. to about 550° C.

In the case of poly(α-methylstyrene) the temperature range ofdecomposition should be limited to about 200° C. to about 500° C.

In the case of polyisobutylene the temperature range of decompositionshould be limited to about 300° C. to about 400° C.

However, the above temperature limits, given as an indication, aredefined for individual polymers and the upper temperature limits formixtures naturally will be dependent on the composition of the polymermixture/co-polymer, the gas atmosphere and the pressure under whichdecomposition is conducted. In the presence of a susceptor, temperaturecontrol is vital to prevent thermal runaway and to avoid substantialdegradation of the monomer/monomers.

The products of step (i) are firstly the monomer or monomers in gaseousform and secondly a small amount of residue, which mainly comprisesinorganic pigments and ash (carbon). The residue may be removed from themicrowave reactor periodically by suitable design of the equipment toprevent ingress of oxygen or air into the reaction zone, or leakage ofmicrowave energy from the reactor.

In the case of poly(ethylene terephthalate), terephthalic acid isrecovered as the solid monomeric form from the microwave cavity itself.

After step (ii) which comprises recovery of the gaseous monomer ormonomers, the gaseous monomer or monomers may be subjected to one ormore of the following steps.

Where two or more monomers are recovered in gaseous or liquid form, themonomers may be separated from one another, for example by distillation.In the case of distillation the gaseous monomers are first condensed.

Thereafter, the monomer or monomers may be condensed by conventionalmeans.

If necessary, the condensed monomer or monomers may be purified byconventional means before being recycled to a polymerization process, orprior to use in the manufacture of other products.

In the case of poly(ethylene terephthalate), the solid terephthalic acidrecovered may be purified by conventional means.

The invention also covers the use of a monomer produced by the processdescribed above in a process for the polymerization of the monomer,optionally with one or more additional monomers, and optionally mixedwith virgin monomer in a suitable ratio, to produce a polymer.

Similarly the invention covers the use of the monomer produced by theprocess described above in the manufacture of other products.

For example, in the case of PMMA, the recovered monomer MMA may be usedin the manufacture of acrylic resins, plastics and fibers, impactmodifiers and processing acids, emulsion polymers, mineral-based sheetpolyesters, polymer concrete and speciality methacrylares such as butylmethacrylate, stearyl methacrylate, decyl methacrylace and 2-ethylhexylmethacrylate.

In the case of polystyrene, the recovered styrene monomer may be used inthe manufacture of acrylonitrile-butadiene-styrene (ABS) resins,styrene-acrylonitrile (SAN) resins, styrene-bucadiene (S/B) copolymerlatexes, unsaturated polyester resins, styrene-butadiene rubber (SBR)elastomers and latexes, styrenated phenols, styrene oxide and styrenatedoils.

In the case of poly(ethylene terephthalate), the recovered and purifiedterephthalic acid may be used in the manufacture of polyester films andfibers, poly(ethylene terephthalate) solid state resin, terephthaloylchloride, dioctyl terephthalate, liquid crystal polymers, amorphousnylons, thermoplastic resins and dimethyl tetrachloroterephthalate, andas a raw material in fine organic chemical synthesis.

The invention will now further be described with reference to theaccompanying drawings which are given by way of example only. Referringto FIGS. 1 and 2, there is shown a laboratory scale microwave apparatus10. The apparatus 10 includes a 6 kW microwave variable power unit 12with an operating frequency of 2.45 GHz. The power unit 12 is fittedwith forward 14 and reflected 16 power gauges. To the power unit 12there is attached a waveguide 18 with a mono-mode cavity 20 located inan opening in the waveguide 18 but designed to prevent any microwaveleakage. The waveguide 18 is fitted with a stub tuner 22 to tune themicrowave energy being supplied to the cavity 20 and an isolator 24 toabsorb the reflected power. The microwave energy is transferred via thewaveguide 18 to the mono-mode cavity 20 where it is absorbed by thepolymer 26. A quartz tube 28 or other suitable microwave transparentvessel holds the polymer 26 inside the microwave cavity 20. The tube 28is fitted with gas tight seals 30 to allow for efficient productrecovery and an effective inert gas atmosphere. The inert gas isintroduced at 32 and is controlled by a calibrated flow meter 34. Thegaseous monomer product is condensed in two liquid nitrogen traps 36 orother suitable equipment. The apparatus 10 is located in a fume hood 38for safety purposes to remove any organic vapours which are notcondensed by the liquid nitrogen gas traps 36.

Referring to FIG. 3, reference 40 generally indicates a modification ofthe mono-mode cavity 20 described in FIGS. 1 and 2, which includesadditional features required to tune the microwave cavity to maximizethe coupling of the microwave energy with the polymer being irradiated.The mono-mode cavity 40 comprises an opening in a waveguide 42 designedto prevent microwave leakage. The waveguide 42 is connected to amicrowave variable power supply 44 which is fitted with reflected 46 andforward 48 power gauges. The waveguide 42 is fitted with an adjustableshort circuit device 50, an adjustable iris 52 and graduated stub tuners54, used to tune the microwave energy supplied to the cavity 40. Byadjusting the position of the standing wave in the waveguide 42, it ispossible to minimize the reflected power and maximize the coupling ofthe microwave energy with the polymer being irradiated.

Referring to FIG. 4 there is shown a laboratory scale microwaveapparatus 60 fitted with a multi-mode cavity 62. The apparatus 60includes a 6 kW microwave variable power unit 64 with an operatingfrequency of 2.45 GHz. The power unit 64 is fitted with reflected 66 andforward 68 power gauges. Attached to the power unit 64 is a waveguide70. The multi-mode cavity 62 comprises a stainless steel cavity designedto contain the microwave energy supplied by the power unit 64. Thewaveguide 70 is fitted with a stub tuner 72 to tune the microwave energybeing supplied to the cavity 62 and an isolator 74 to absorb thereflected power and thereby protect the magnetron. The microwave energyis transferred via the waveguide 70 to the cavity 62 where it isabsorbed by a polymer 76. The polymer 76 may be placed directly in thecavity 62. Alternatively, the polymer is placed on a ceramic supportinside a gas in-tight, microwave transparent container 78. Where themonomer is in gaseous form, the product is swept out of the microwavetransparent container 78 and condensed outside the microwave cavity 62through a gas outlet 80. The container 78 is constantly purged with aninert gas through a gas inlet 82. The whole apparatus is enclosed in afume hood 84 for safety purposes.

Referring co FIG. 5(a), there is shown a bench scale (1.5-2 kilogram perhour semi-continuous) microwave apparatus 90 fitted with a mono-modecavity 92. The apparatus 90 includes a 6 kW microwave variable powerunit 94 (FIG. 5(b)) with an operating frequency of 2.45 GHz. (On anindustrial scale a frequency of 915 MHz is more favourable.) The powerunit 94 is fitted with forward 96 and reflected 98 power causes as shownin FIG. 5(b). Attached to the power unit 94 is a waveguide 100.

The mono-mode cavity 92 comprises a cylindrical stainless steel cavity102. The mono-mode cavity 92 is specially designed so as to maximize theelectric field in the lower section of the reactor 92 and the cavity isspecially tuned by the creation of a small iris 104 in the center of thecavity wall where the waveguide 100 makes contact with the wall. Thedimensions of the iris 104 are critical to maixmize energy couplingwithin the cavity 92. To prevent substantial quantities of the gaseousproduct from diffusing into and condensing in the waveguide 100, whichcould cause a fire, a quartz window (transparent to microwave energy)106 is sealed in a flange 108, with a recess and special seals, in thesection 110 of the waveguide 100 in contact with the reactor 92. Thissection 110 of waveguide 100 is jacketed to allow cooling (water orother suitable coolant). A nitrogen purge inlet 112 is located betweenthis section 110 and the cavity wall 102 to prevent condensation of thegaseous product on the quartz window 106. As a precaution, in the eventthat the quartz window 106 should be damaged, a second quartzwindow/flange system 114 is located further up the waveguide 100, toprotect the magnetron. The section of the waveguide between the quartzwindows 106 and 114 is purged with nitrogen through an inlet 112. Thenitrogen gas exists at outlet 116 and may be tested for the presence ofmonomer by a suitable gas sensing device (e.g a Drager tube).

A polymer 118 is fed to the cavity 92 via a feed hopper 120. The feedhopper 120 is connected to a feed tube 122, sealed between two valves124 and 126. A nitrogen inlet 128 allows this tube 122 to be pureed ofoxygen during feeding. The operation involves opening the upper valve124 while the lower valve 126 is closed, filling the tube 122 withpolymer while purging with a nitrogen flow (inlet 128) and then closingthe upper valve 124. The lower valve 126 is then opened to allow thepolymer to fall through a section 130, heated with heating tape, beforefailing through a choke 132 purged with nitrogen through an inlet 134.The lower valve 126 is closed immediately thereafter, to prevent thediffusion of gaseous product into the feed mechanism, which may causestickiness and result in blockage of the tube 122. The choke 132 isdesigned with suitable dimensions to prevent the leakage of microwaveradiation from the cavity 92.

The polymer 118 falls through into the cavity 92, where it may bepreheated (although this is not necessary in the case of some polymers,e.g PMMA) prior to irradiation with microwave energy by any conventionalheating means, in this case a heating tape 136, set at a predeterminedtemperature. Where the monomer is recovered in gaseous form, the gaseousproduct arising from the decomposition of the polymer exits the reactor92 through an outlet 138 (of suitable dimensions so as not to permitmicrowave leakage from the cavity) and passes along a heated line to asuitably designed and chilled condenser 140, prior to being collected ina suitable vessel 142 which may or may not be cooled further. Samplescan be capped from the condensate collection vessel 142 at a samplingpoint 144, to allow the product purity and composition to be determinedperiodically during the reaction. Nitrogen puree gas exiting the systemvia the storage/collection vessel 142 at the exit point 146 is monitoredcontinuously by an oxygen meter for safety purposes.

For safety, a pressure relief valve 148 and a pressure monitoring device150 (e.g., manometer or pressure gauge) may be connected to the cavity92 on an upper flange 152 on the cavity 92, provided that the dimensionsare such that no leakage of microwaves occurs.

A thermocouple 154 is located in the gas outlet pipe 138 to monitor thetemperature of the gaseous product prior to condensation. Anotherthermocouple 156 is located in the lower region of the cavity 92 (in theregion of the polymer) to monitor the temperature during decomposition.The feed rate is controlled by monitoring the reflected power 98 (seeFIG. 5(b)) and matching the feed rate to the decomposition rate in thecavity. The entire reactor 92 is insulated by encapsulating it in asuitable refractory (insulating) material such as Fibrefrax™ 158.

Referring to FIG. 6, there is shown a 1000 ml multi-purpose microwaveapparatus fitted with a non-resonant cavity 160. The apparatus includesa 6 kW variable microwave power unit, with an operating frequency of2.45 GHz. The unit is fitted with forward and reflected power gauges.Attached to the power unit is a waveguide 162. The waveguide 162 isfitted with a non-resonant cavity 160 and an adjustable short circuitdevice 164. The adjustable short circuit device 164 is used todynamically adjust the impedance match as the load changes. To preventdiffusion of degradation products into the waveguide 162 and shortcircuit device 164, the cavity 160 is fitted at each end with a pair ofquartz waveguide windows 166. Each quartz window 166 is sealed inspecially designed flanged fittings, with a recess and special seals.Nitrogen inlets 168 purge the cavity 160 of oxygen and the section ofwaveguide 162 between each pair of waveguide windows 166, respectively.A methyl methacrylate Drager tube 170 (or other suitable gas sensingdevice in the case of other polymers) is connected to the nitrogenoutlet on the section of waveguide 162 between each pair of waveguidewindows 166. Breakage of a quartz window 166 closest to the cavity 160is indicated by a change in color of the Drager tube 170 (caused forexample in the case of PMMA, by the presence of methyl methacrylate).The gaseous product arising from the decomposition of the polymer exitsthe cavity through an outlet 172 (of suitable dimensions so as not topermit microwave leakage from the cavity) and passes through a condenser174, prior to collection in a suitable collection vessel 176. A gas bomb178 is connected to the collection vessel 176, to allow a headspace gassample to be collected for analysis. The line between the cavity 160 andthe condenser 174 is heated by heating tape, to prevent condensation ofmonomer in the line to the condenser 174. A thermocouple 180 is locatedin the lower region of the cavity 160 to monitor the temperature duringdepolymerization. A thermocouple 182 is located in the gas outlet pipe172 to monitor the temperature of the gaseous products prior tocondensation.

For safety, a manometer 184 is connected to the nitrogen purge inletline 168 to monitor pressure and allow release of pressure, should ablockage occur down stream from the outlet port 172. An oxygen meter isconnected to the gas exit port to monitor oxygen levels in the outletgases. Nitrogen regulators regulate the flow of nitrogen to the variousnitrogen purge lines.

The cavity is loaded with approximately 700 ml of polymer, prior toassembly to the waveguide. The system then is pureed with nitrogen andthe polymer then irradiated with microwaves, once the levels of oxygenin the outlet gases are less than 0.5%. The adjustable short circuitdevice is adjusted during the run to minimize the reflected power. Aheadspace gas sample is collected (i.e from the space above thecondensate) in a gas bomb, once it is evident that condensation isoccurring. The run is stopped, once the temperatures measured in thecavity and gas outlet pipe begin to drop rapidly (indication thatdepolymerization has ceased) and no further condensate is produced (asmonitored by the level of condensate in the collection vessel).

This system (FIG. 6) represents a multi-purpose microwave apparatus fortesting a range of different polymers and polymer mixtures. In theexamples provided (non-resonant cavity), this system was used todemonstrate the principle of depolymerization only and was not optimizedfor any particular polymer. Suitable adjustment of the cavity andoptimization of conditions by one skilled in the art would permitmaximum depolymerization of the polymer, and recovery of themonomer/monomers, for a particular polymer or polymer system.

The invention will now be further illustrated by various examples, whichwere carried out in the apparatus mentioned above.

All recoveries are calculated as the mass of monomer condensed as apercentage of the mass of polymer depolymerization. All yields arecalculated as the mass of monomer condensed as a percentage of the totalmass of polymer added to the cavity.

EXAMPLE 1

Application of the Process of the Invention to PMMA

Example 1(a)

Decomposition of poly(methyl methacrylate) in the Mono-mode Cavity(laboratory scale)

A sample of powdered poly(methyl methacrylate) (±1 g) was irradiated inthe mono-mode microwave cavity with 200 W of forward power for 60seconds (10% of the power was not absorbed by the material). The productcondensed was analyzed for purity by a quantitative gas chromatography(GC) analysis and indicated a product purity of 99%. The polymer massloss was 96% and the monomer yield was 86%.

Example 1(b)

Decomposition of Powdered Cast poly(methyl methacrylate) in a Mono-modeCavity (laboratory scale)

A sample of powdered cast poly(methyl methacrylate) (<500 μm, 3.1 g)mixed with approximately 1% (m/m) carbon powder (as a microwavesusceptor), was irradiated in a mono-mode microwave cavity (FIG. 1) with1 kW of forward power for 2 minutes. The mass of the residue in thecavity, after irradiation had ceased, was determined and the polymermass loss then calculated to be 97%. The gaseous product was analysed byGas Chromatography--Mass Spectrometry (GCMS) and found to containpredominantly MIMA, with traces of styrene, indene, naphthalene,dimethyl terephthalate and dibutyl phthalate.

Example 1(c)

Decomposition of poly(methyl methacrylate) in a Mono Mode Cavity (benchscale reactor)

A sample of poly(methyl methacrylate) (PMMA) (220 g) was loaded into themicrowave cavity (FIG. 5). The microwave reactor was heated slowly (over2.5 hours) to a maximum of 200° C., using a heating tape wrapped aroundthe cavity. This was done both to avoid thermal shock to the ceramicinsert, and also to preheat the polymer to improve microwave coupling tothe material. The forward power was then increased slowly over a 10minute period to a maximum of 4 kW. During the experiment a further1.647 g of PMMA was added to the reactor in small increments. whilstirradiating continuously (at 4 kW) over a period of 120 minutes.(Forward and reflected power and reaction time were not optimized duringthe experiment). A mass loss of 99.3% was, recorded.

An analysis of the residue in the cavity ("ash" or "dross") afterirradiation was stopped, using X-ray fluorescence, showed the followingcomposition of the "ash" or "dross", which reflects the inorganicpigments and additives in the original PMMA:

Cd(0.6%); Sr(<0.5%); Fe(0.5%); Cr(0.5%); Ti(16%); Ca(50.5%); K(<0.5%);S(4%); Al(4%); O(28%). (Other elements (Ni, Zn, Se, Cu, Si, Nd, Sb) wereless than 0.1%). (Balance carbon)

(Note: During early developmental work (prior to design of the quartzwindow system), a high purity cylindrical alumina insert was used toline the inside of the stainless steel cavity. The narrow gap betweenthe stainless steel wall and the ceramic insert was sealed at each endusing Teflon® seals. The function of the ceramic insert (transparent tomicrowaves at these temperatures) was to prevent diffusion of monomervapors into the waveguide and, thus to protect the magnetron. In view ofthe practical difficulties associated with this system, the improveddesign with a system of quartz microwave windows as described by FIG. 5was developed and adopted in all subsequent bench scale work describedherein).

Example 1(d)

Decomposition of poly(methyl methacrylate) in a Mono Mode Cavity (benchscale reactor) with Conventional Preheating

During early experimental work (i.e prior to design optimization), aseries of experiments was conducted where the initial temperature of thepolymer (PMMA) was varied from 73° C. (run 1) to 95.3° C. (run 2), 134°C. (run 3) and finally 192° C. (run 4), and the sample irradiated underthe same conditions of forward power (2 kW) for 40 minutes, with furtheradditions of PMMA after approximately 10 minutes (50 g), 20 minutes (50g) and 30 minutes (50 g), during each run. The following mass losseswere recorded after the same total reaction time, demonstrating thebeneficial effect of preheating the sample to >100° C.

    ______________________________________                                        Run 1 (73° C.)                                                                         46.0%                                                         Run 2 (95.3° C.)                                                                       41.9%                                                         Run 3 (134.3° C.)                                                                      80.1%                                                         Run 4 (192° C.)                                                                        81.9%                                                         ______________________________________                                    

It appears that a critical temperature exists (>100° C.), above which nofurther enhancement in the rate of initial mass loss is gained byincreasing the temperature further. Similarly, preheating to anytemperature below 95° C. gives similar initial low mass losses of aroundonly 40-45%.

During subsequent optimization of the reactor design and other processconditions, it was found to be unnecessary to preheat PMMA. Howeverpreheating may still have a beneficial effect for other polymers.

Example 1(e)

Decomposition of poly(methyl methacrylate) in the Multi-mode Cavity(laboratory scale)

A sample of poly(methyl methacrylate) chips (±330 g) was irradiated inthe multi-mode microwave cavity with 1 600 W of forward power for 23minutes. The polymer mass loss was 98% and the product purity was 97%.(Forward and reflected power and reaction time were not optimized).

Example 1(f)

Decomposition of Clear Cast poly(methyl methacrylate) in a Mono-modeCavity (bench scale reactor)

The walls of the microwave reactor were pre-heated to a maximum of 200°C., using a heating tape wrapped around the cavity, to preventcondensation of monomer on the walls of the cavity. The reactor systemwas purged with nitrogen, and the oxygen levels monitored by means of anoxygen meter placed at the exit on the collection vessel. The forwardpower was set at 1 kW and a sample of clear cast poly(methylmethacrylate) (130 g) was fed incrementally via the valve system intothe microwave cavity over a period of about 2 minutes (FIG. 5). Duringthe experiment, a further 5-6 kg of PMMA was added to the reactor insmall increments (so as to minimize reflected power), whilst irradiatingcontinuously (at 1 kW) over a period of 3.54 hours. The reactiontemperature recorded during the depolymerization- was in the range 320°C. to 390° C. An average depolymerization rate of 1.4 kg/hr wascalculated. A total polymer mass loss of 98-99% was recorded. Theaverage purity of the recovered methyl methacrylate (MMA) monomer was95% (standard deviation 1%), and the overall MMA recovery and MMA yield,93% (standard deviation 1%) and 90% (standard deviation 0.5%),respectively.

The actual energy utilization within the cavity was calculated as 0.714kWhr/kg PMMA (does not take into account mains-to-magnetron electricalconversion efficiency).

An analysis of the residue in the cavity after irradiation was ceased,using Nuclear Magnetic Resonance Spectrometry (NMR), showed the presenceof PMMA (only) with a molecular weight of approximately 16000.

Example 1(g)(1)

Decomposition of Pigmented or Dyed poly(methyl methacrylate) in aMono-mode Cavity (bench scale reactor)

Using procedures identical to those conducted for Example 1(f), samplesof pigmented (1 662 g) and dyed (5 053 g) PMMA were depolymerized inseparate experiments over periods of 1 hour and 3 hours, respectively.

The reaction temperature recorded during the depolymerization was in therange 330° C. to 390° C. Mass losses, MMA product purities (determinedby GC analysis), MMA recoveries and MMA yields were as follows:

    ______________________________________                                                          MMA     MMA        MMA                                               Mass Loss                                                                              Purity  Recovery   Yield                                             (%)      (%)     (%)        (%)                                      ______________________________________                                        Pigmented  94         98.1    94       88                                     PMMA                                                                          Dyed PMMA  98         94.3    93       91                                     ______________________________________                                    

Example 1(g)(2)

Repolymerization of a Monomer to its Polymer

A sample of MMA prepared by the depolymerization of pigmented PMMA (asdescribed in Example 1(g)(1)) was distilled at atmospheric pressure. 700g of the distillate was activated and gently heated for 30 minutes toproduce a partially polymerized, low viscosity gel (syrup). The gel wasdoped with various agents including a splitting aid and a peaksuppressor prior to repolymerization to form a sheet of dimensions 4mm×356 mm×356 mm. The latter PMMA sheet (produced from 100%depolymerized PMMA), was tested for ease of splitting from the castingmold, edge and sheet color, clarity, heat test quality and reducedviscosity. These results were found to be comparable to the resultsobtained for a typical commercially produced PMMA sheet.

Example 1(h)

Decomposition of Clear Cast poly(methyl methacrylate) in theNon-resonant Cavity

A sample of clear cast poly(methyl methacrylate) chips (381 g) wasirradiated in the multi-purpose, non-resonant microwave cavity (FIG. 6)with 1 kW of forward power for 20 minutes. The reaction temperaturerecorded during the depolymerization was in the range 360° C. to 385° C.The mass of the residue in the cavity, after irradiation had ceased, wasdetermined and the polymer mass loss then calculated to be 58%. The MMAproduct purity was determined by GC analysis to be 98.7%, giving anoverall MMA recovery and MMA yield of 80% and 46%, respectively.

Example 1(i)

Decomposition of Clear Extruded poly(methyl methacrylate) in theNon-resonant Cavity

A sample of clear extruded poly(methyl methacrylate) chips (500 g) wasirradiated in the multi-purpose, non-resonant microwave cavity (FIG. 6),with 1 kW of forward power for 36 minutes. The reaction temperaturerecorded during the depolymerzation was in the range 300° C. to 400° C.A headspace was sample was taken after 14 minutes of continuousmicrowave irradiation. The gaseous product was analysed by GCMS andfound to contain: carbon dioxide; 2-methyl-1-propene; methyl acrylateand MMA. The mass of the residue in the cavity, after irradiation hadceased, was determined and the polymer mass loss calculated to be 86%.The condensate was analysed by GC and found to contain 93.4% MMA, withan overall MMA recovery and MMA yield of 60% and 52%, respectively. Thecondensate also was analyzed by GCMS analysis and found to contain minorimpurities of: ethenyl methacrylate; methyl dimethylpentenoate; butylmethacrylate; dimethyl methylenebutanedioate; dimethyl2-methylpentanedioate; dimethyl (methyl-propenyl)propanedioate anddimethyl-1,4-cyclohexane dicarboxylate.

Example 1(j)

Decomposition of High Impact poly(methyl methacrylate) in theNon-resonant Cavity

A sample of high impact poly(methyl methacrylate) chips (480 g) wasirradiated in the multi-purpose, non-resonant microwave cavity (FIG. 6),with 0.5 kW of forward power for 90 minutes. The reaction temperaturerecorded during the depolymerization was in the range 300° C. to 415° C.A headspace gas sample was taken after 17 minutes of continuousmicrowave irradiation. The gaseous product was analyzed by GCMS andfound to contain: carbon dioxide; 2-methyl-1-propene; ethyl acrylate;MMA and methyl-2-methylpropanoate. The mass of the residue in thecavity, after irradiation had ceased, was determined and the polymermass loss then calculated to be 85%. The condensate was analyzed by GCand found to contain 71.7% MMA, with an overall MMA recovery and MMAyield of 47% and 40%, respectively. The condensate also was analyzed byGCMS and found to contain minor quantities of: ethyl methacrylate;styrene; butyl acrylate; methyl dimethylpentenoate and butylmethacrylate.

Example 1(k)

Decomposition of poly(methyl) methacrylate) Swarf in a Mono-mode Cavity(laboratory scale)

A sample of poly(methyl methacrylate) swarf ("saw-dust") (2.1 g) mixedwith approximately 1% (m/m) carbon powder (as a microwave susceptor),was irradiated in a mono-mode microwave cavity (FIG. 1), with 1 kW offorward power for 2 minutes. The mass of the residue in the cavity,after irradiation had ceased, was determined and the polymer mass lossthen calculated to be 98%. The gaseous product was analyzed by GCMS andfound to contain predominantly MIMA, with traces of styrene, naphthaleneand dibutyl phthalate.

Example 1(l)

Decomposition of poly(methyl methacrylate) Gel in a Mono-mode Cavity(laboratory scale)

A sample of poly(methyl methacrylate) gel (partially polymerized MMAfrom the plant) (0.8 g), mixed with approximately 1% (m/m) carbon powder(as a microwave susceptor), was irradiated in a mono-mode microwavecavity (FIG. 1), with 1 kW of forward power for 2.5 minutes. The mass ofthe residue in the cavity, after irradiation had ceased, was determinedand the polymer mass loss then calculated to be 84% The gaseous productwas analyzed by GCMS and found to contain predominantly MMA, with tracesof xylene, styrene, 2-methyl propanenitrile and methyl2-methylbutanoate.

Example 2

Application of the Process of the Invention to Physical Mixtures ofPolymers

Example 2(a)(1)

Decomposition of a Physical Mixture of Polymers, where Both PolymericSubstances Decompose to their Corresponding Monomers

A sample of clear cast poly(methyl methacrylate) chips (231 g) and clearpolystyrene pellets (231 g), was irradiated in a multi-purpose,non-resonant microwave cavity (FIG. 6), with 1 kW of forward power for60 minutes. The reaction temperature recorded during thedepolymerization was in the range 330° C. to 400° C. A headspace gassample was taken after 22 minutes or continuous microwave irradiation.The gaseous product was analyzed by GCMS and found to contain: MMA;2-methyl-1-propene; carbon dioxide, and traces of methyl2-methylpropanoate; toluene and styrene. The mass of the residue in thecavity, after irradiation had ceased, was determined and the totalpolymer mass loss calculated to be 70%. The condensate was analyzed byNMR and found to contain 29% (m/m) MMA and 64% (m/m) styrene, giving ayield of MMA and styrene of 31% and 67%, respectively. The condensatealso was analyzed by GCMS and found to contain minor quantities ofmethyl styrene, styrene dimer and trimer, as well as traces of toluene.

Example 2(a)(2)

Separation of Two Monomers Recovered in Liquid Form, by Distillation

The condensate produced as described in Example 2(a)(1), and containing29% (m/m) MMIA and 64% (m/m) styrene, was distilled under a vacuum of60-70 kPa. During the distillation, the temperature was increasedincrementally from 25 to 150° C., over approximately 6 hours. The firstfraction collected was analyzed by NMR and found to contain 84% (m/m)MMA and 6% (m/m) styrene. The final fraction, also analyzed by NMR, wasfound to contain 72% (m/m) styrene and 18% (m/m) MMA. Optimization ofthe distillation procedure, would improve the separation of themonomers.

Example 2(b)

Decomposition of a Physical Mixture of Polymers, where Both PolymericSubstances Decompose, but Only One, to its Corresponding Monomer

A mixture of clear cast poly(methyl methacrylate) chips (209 g) andclear polyethylene beads (209 g) was irradiated in a multi-purpose,non-resonant microwave cavity (FIG. 6), with 1 kW of forward power for45 minutes. The reaction temperature recorded during thedepolymerization was in the range 300° C. to 390° C. A headspace gassample was taken after 25 minutes of continuous microwave irradiation.The gaseous product was analyzed by GCMS and found to containpredominantly MMA, 2-methyl-1-propene and carbon dioxide. The mass ofthe residue in the cavity, after irradiation had ceased, was determinedand the total polymer mass loss calculated to be 38%. The condensate wasanalyzed by GC and found to contain 97.6% MMA. The condensate also wasanalyzed by GCMS and found to contain minor quantities of2-methylbutanoate ester, esters of pentane and hexane, styrene, styrenedimer and C₁₄ H₂₈, and C₁₆ H₃₂.

Example 2(c)

Selective Decomposition of a Physical Mixture of Polymers, where theDecomposition Temperature and the Dielectric Loss Factors areSignificantly Different

A sample of clear cast poly(methyl methacrylate) chips (243 g) andpolytetrafluoroethylene powder (243 g), was irradiated in amulti-purpose, non-resonant microwave cavity (FIG. 6), with 1 kW offorward power for 40 minutes. The reaction temperature recorded duringthe depolymerization was in the range 300° C. to 390° C. A headspace gassample was taken after 15 minutes of continuous microwave irradiation.The gaseous product was analyzed by GCMS and found to contain MMA,2-methyl-1-propene and carbon dioxide. The mass of the residue in thecavity, after irradiation had ceased, was determined and the totalpolymer mass loss then calculated to be 36%. The condensate was analyzedby GC and found to contain 99.6% MMA.

Example 3

Application of the Process of the Invention to polystyrene (PS)

Example 3(a)

Decomposition of polystyrene in the Mono-mode Cavity (laboratory scale)

A sample of polystyrene (±0.5 g) and carbon (20% by mass) wereirradiated in the mono-mode microwave cavity (FIG. 1) with 1 kW offorward power for 60 seconds. Styrene monomer was recovered from thegases evolved. (Forward and reflected power and reaction time were notoptimized).

Example 3(b)

Decomposition of polystyrene in a Non-resonant Cavity

A sample of clear polystyrene pellets (481 g) and 10% (m/m) carbonpellets, was irradiated in a multi-purpose, non-resonant microwavecavity (FIG. 6), with 0.5 kW of forward power for 15 minutes followed by1 kW of forward power for 10 minutes. The reaction temperature recordedduring the depolymerization was in the range 200° C. to 290° C. Aheadspace gas sample was taken after 24 minutes of continuous microwaveirradiation. The gaseous product was analyzed by GCMS and found tocontain styrene, carbon dioxide, benzene, ethylbenzene and toluene. Themass of the residue in the cavity, after irradiation had ceased, wasdetermined and the polymer mass loss then calculated to be 21%. Thecondensate was analyzed by NMR and found to contain approximately 60%(m/m) styrene, 30% (m/m) styrene trimer and styrene dimer, 6% (m/m)ethylbenzene and 4% (m/m) toluene. The condensate also was analyzed byGCMS and found to contain small quantities of (1-methylethyl) benzene;(1-methylethenyl) benzene; and benzene. The latter compounds were notdetected by NMR, as the signals for these compounds were masked by thesignals of other compounds, present in larger quantities.

Example 4

Application of the Process of the Invention to poly(ethyleneterephthalate) (PET)

Example 4(a)

Decomposition of poly(ethylene terephthalate) in the Mono-mode Cavity

A sample of poly(ethylene terephthalate) (2.01 g)mixed with (0.09 g)carbon (as a microwave susceptor, 4.36% m/m) was irradiated in amono-mode cavity using a forward power of 1 kW and a total irradiationtime of 5 minutes (neither power nor reaction time was optimized). Amass loss of 66.7% was recorded. The gaseous product was analyzed andfound to contain carbon dioxide, acetaldehyde and benzene as the majorproducts. The residue, was found to contain predominantly terephthalicacid, one of the valuable monomers used in the manufacture of PET, witha small amount of unreacted PET.

Example 4(b)

Decomposition of poly(ethylene terephthalate) in a Non-resonant Cavity

A sample of clear poly(ethylene terephthalate) (PET) pellets (717 g) and10% (m/m) carbon pellets, was irradiated in a multi-purpose,non-resonant microwave cavity (FIG. 6) with 0.5 kW of forward power for30 minutes. The reaction temperature recorded during thedepolymerization was in the range 400° C. to 470° C. A headspace gassample was taken after 12 minutes of continuous microwave irradiation.The gaseous product was analyzed by GCMS and found to containpredominantly carbon dioxide and acetaldehyde. Minor quantities ofbenzene, ethylbenzene, toluene and styrene were also found to be presentin the gas sample. As expected, no condensate was formed during thisrun. The solid product deposited on the walls of the cavity (outside thereaction zone), was analyzed by NMR and found to contain approximately44% (m/m) terephthalic acid and 56% (m/m) hydroxy (vinyloxycarbonyl)benzoic acid.

Example 5

An Illustration of Polymers which Do Not Degrade to Monomers

Example 5(a)

Decomposition of polyethylene in a Mono-mode Cavity (laboratory scale)

A sample of polyethylene (2.4 g) mixed with approximately 14% (m/m)carbon powder (as a microwave susceptor), was irradiated in a mono-modemicrowave cavity (FIG. 1), with 2 kW of forward power for 2 minutes. Thegaseous product was analyzed by GCMS and found to contain propene.hexene, benzene, heptene, as major products and octene, nonene, styrene,1-propyl benzene, and hydrocarbons with 10 to 23 carbons, as minorproducts.

Example 5(b)

Decomposition of polypropylene in a Mono-mode Cavity (laboratory scale)

A sample of polypropylene (1.5 g) mixed with approximately 15% (m/m)carbon powder (as a microwave susceptor), was irradiated in a mono-modemicrowave cavity (FIG. 1), with 1 kW of forward power for 30 seconds.The gaseous product was analyzed by GCMS and found to contain propene,hexene, benzene and nonene, as major products, toluene and styrene asminor products, and traces of 1 propynyl benzene, 2 dodecene,naphthalene, C₁₅ H₃₀, C₁₆ H₃₂, C₁₂ H₁₀, C₁₄ H₁₀ and C₁₆ H₁₀.

The process of the invention has various advantages over the knownconventional process for the decomposition of polymers. The advantagesof the process of the invention include increased product purity,potentially eliminating the need for ocher downstream processing steps,minimal environmental impact, greater energy efficiency due to directvolumetric heating of the polymer, improved occupational health andsafety features, and reduced solid waste accumulation. The improvedenergy efficiency and reduced effluent volumes (such as wash effluentand lead dross in the case of PMMA) provide significant operational costadvantages to the microwave process over alternative and moreconventional depolymerization processes, despite an initially highercapital investment.

In particular the invention provides a process, which is of particularuse with poly(methyl methacrylate) as the polymeric species. Based onextensive bench scale (1.5 kg/hr) studies on PMMA, a comparison of thecosts of production (variable, excluding cost of scrap PMMA, and fixedoperating costs, excluding depreciation) for the microwave and lead bathprocesses shows savings of 74% in the microwave process over the leadbath. This is due to the fact that energy costs can be reduced by up to61%, while costs of chemicals can be reduced by 87%, by eliminatingdownstream processing steps (e.g washing). Furthermore, elimination ofsolid and aqueous effluents considerably reduces the costs of effluentdisposal and environmental monitoring by up to 94%. Including the costof scrap PMMA, this translates to savings of 11% on the cost ofproduction for the microwave process, over the lead bath.

What is claimed is:
 1. A process for decomposing a polymer which iscapable of undergoing thermal depolymerization to its monomer ormonomers, the polymer being selected from the group consisting ofpoly(methyl methacrylate), polytetrafluoroethylene, polystyrene,poly(ethylene terephthalate), poly(α-methylstyrene) and polyisobutylene,and for recovery of at least one of the monomers, includes the stepsof:(i) subjecting the polymer in solid, eel, partially molten or moltenform to microwave heating for a time and at a temperature sufficient todecompose the polymer to produce the monomer or monomers in gaseous,liquid or solid form, without substantial decomposition of the monomeror monomers; and (ii) recovering at least one of the monomer ormonomers.
 2. A process according to claim 1 wherein the process includesthe step of:(iii) where two or more monomers are recovered in step (ii),separating the monomers from one another.
 3. A process according toclaim 1 wherein the process includes the step of:(iv) where the monomeror monomers are in gaseous form, condensing the monomer or monomers. 4.A process according to claim 1 wherein the process includes the stepof:(v) purifying the condensed monomer or monomers.
 5. A processaccording to claim 1 wherein prior to step (i) the polymer is preheatedto a suitable temperature.
 6. A process according to claim 1 wherein instep (i) the polymer is mixed with a microwave absorber or susceptor. 7.A method according to claim 1 wherein step (i) is carried out under aninert atmosphere.
 8. A process according to claim 1 wherein in step (i)the microwave heating is carried out in a mono-mode, a multi-mode, or anon-resonant cavity of a microwave reactor.
 9. A process according toclaim 1 wherein the polymer is selected from the group consisting ofpoly(methyl methacrylate), polytetrafluoroethylene, poly(∝-methylstyrene), polystyrene and, polyisobutylene.
 10. A processaccording to claim 9 wherein the polymer is poly(methyl methacrylate).11. The use of a monomer produced by a process according to claim 1, ina process for the polymerization of the monomer, optionally with one ormore additional monomers, and optionally mixed with virgin monomer, toproduce a polymer.
 12. A process for the production of a polymer from amonomer produced by the process of claim 1 wherein the monomer ispolymerized, optionally with one or more additional monomers, andoptionally mixed with virgin monomer.